Update alternative-text-for-images.mdx (primer#635)

minor grammatical errors.

content/guides/accessibility/alternative-text-for-images.mdx

@@ -1,11 +1,11 @@
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 title: Alternative text for images
-description: Alternative text on images allows assistive technology like screen readers to understand the purpose of an image on a page or allow them to skip it if purely decorative.
+description: Alternative text on images allows assistive technology like screen readers to understand the purpose of an image on a page or allows them to skip it if purely decorative.
 ---
 
 ## Overview
 
-> Text alternatives are a primary way for making information accessible because they can be rendered through any sensory modality (for example, visual, auditory or tactile) to match the needs of the user. Providing text alternatives allows the information to be rendered in a variety of ways by a variety of user agents. For example, a person who cannot see a picture can have the text alternative read aloud using synthesized speech.
+> Text alternatives are a primary way for making information accessible because they can be rendered through any sensory modality (for example, visual, auditory, or tactile) to match the needs of the user. Providing text alternatives allows the information to be rendered in a variety of ways by a variety of user agents. For example, a person who cannot see a picture can have the text alternative read aloud using synthesized speech.
 >
 > Source: https://www.w3.org/TR/UNDERSTANDING-WCAG20/text-equiv-all.html